1. Field of the Invention
The present invention relates to an integrated optical semiconductor device used in optical fiber communication or optical information processing.
2. Description of the Related Art
In optical fiber communication or optical information processing, a light source such as a semiconductor laser has been generally used. However, in order to establish a desired information system, the phase and frequency of light emitted from the semiconductor laser must be controlled with high precision. For this reason, an optical circuit is formed on a substrate by an optical waveguide, and an optical signal must be processed, or a semiconductor optical waveguide and a photodetector must be integrated. In addition, it has been known that a photodiode is arranged at a position opposite to the radiation direction of the semiconductor laser to control or monitor the operation of the semiconductor laser. In a conventional technique, a semiconductor laser and a photodiode are independently provided, and they are fixed on a base to be opposed to each other.
In recent years, an optical semiconductor device integrated by forming a laser diode and a photodiode on the same semiconductor substrate has been proposed to obtain the most advanced functions, mass-production, and low cost. The integrated optical semiconductor device, as shown in FIG. 9, includes a semiconductor laser and a photodiode formed on an insulating semiconductor substrate 25. The semiconductor laser consists of an n-type InP clad layer 24, an InGaAsP active layer 23, a p-type InP clad layer 22, a p-type InGaAsP contact layer 21, a p-side electrode 20, and an n-side electrode 26, and the photodiode consists of an InGaAsP layer 30 serving as an light absorption layer, an n-type InP layer 31, a p-type InP layer 29, a p-type InGaAsP contact layer 28, an n-side electrode 32, and a p-side electrode 27. Since this photodiode is formed in the same process as that of the semiconductor laser, the composition of the photodiode is the same as that of the semiconductor laser. A positive potential and a negative potential are applied to the n-side electrode 32 and the p-side electrode 27 in the photodiode, respectively. For operating the semiconductor laser, a positive potential and a negative potential are applied to the p-side electrode 20 and the n-side electrode 26, respectively, to inject carriers to the active layer 23. The semiconductor laser and the photodiode are separated from each other by a groove 33 formed by a dry etching technique such as RIE. The photodiode serving as a photodetector, as described above, is made as follows. That is, the semiconductor layers for providing the semiconductor laser are formed on the semiconductor substrate 25 and etched down to the substrate 25 to form the groove 33. A part of the semiconductor layers separated by the groove 33 is then used as the photodiode. The photodiode, therefore, has the same structure as that of the semiconductor laser. In the semiconductor laser, the multilayer film including the active layer 23 and the clad layers is formed to provide a stripe-shaped structure, and a current blocking layer is grown on the substrate at both sides of the stripe-shaped structure in parellel with an optical axis. In order to provide the current blocking layer, after the multilayer film is provided, the stripe-shaped active region and the surface of the photodiode are covered with an insulating film such as an SiO.sub.2 film or an Si.sub.3 N.sub.4 film. Thereafter, the multilayer film is selectively removed to expose the substrate 25. Semiconductor layers are grown on the exposed semiconductor substrate 25 to form a current blocking layer. Finally, the electrodes are provided to complete the integrated semiconductor device. However, with the above arrangement, since the absorption layer 30 of the photodiode has the same composition as that of the active layer 23 of the semiconductor laser, it has a small absorption coefficient of the laser beam, and the photodiode having a desired sensitivity cannot be obtained. In addition, since the thickness of the absorption layer 30 has the same thickness as that of the active layer 23, and is thin i.e., about 0.1 .mu.m, it cannot sufficiently receive the laser beam emitted from the semiconductor laser. Furthermore, since the insulating substrate 25 is used, the electrodes cannot easily provided because they must be separately provided for elements on the substrate. Therefore, with the above arrangement, an optical semiconductor device having the most advanced functions, mass production, low cost cannot be obtained.